I am primarily a Microsoft platform developer. Microsoft makes a great IDE in the form of Visual Studio.Net and has a great language in C#, which is getting better with C# 2.0.
However, I must say, at least until IE 7 becomes more visible and out of beta, that Mozilla’s Firefox browser provides better tools for web developers through Firefox’s extensions for developers.
I especially like the Web Developer extension. Sure beats the Alt+Tab dance between the browser you are viewing your changes with and your CSS editor!